The invention concerns intervertebral disc prostheses.
The document EP-0,277,282 discloses such a prosthesis comprising two plates which are intended to come into contact with the vertebral plates of the vertebrae adjacent to the disc which is to be replaced, and a cushion interposed between the plates and comprising a compressible body forming a chamber for a liquid. However, wear can occur between the compressible body and the plates, leading in particular to the emission of solid particles and to their dispersion in the human body. Moreover, although the mechanical behavior of this prosthesis comes close to that of a healthy natural intervertebral disc, it is desired to make available a prosthesis which in a different way comes close, or even closer, to the behavior of a normal disc.
It is an object of the invention to make available a prosthesis which generates less wear and which has different mechanical behavior.
With a view to achieving this object, the invention provides an intervertebral disc prosthesis comprising two plates, and a cushion interposed between the plates and having a chamber filled with fluid and a compressible body, where the body is so shaped as to be able to take up a position in which it is in contact with at most one of the plates when the prosthesis is subjected to compression tending to bring the plates closer to each other.
This compression will be able to have an intensity equal to 3000 N.
The invention also provides an intervertebral disc prosthesis comprising two plates, and a cushion interposed between the plates and having a chamber filled with fluid and a compressible body, where the body is so shaped as to be able to take up a position in which it is in contact with at most one of the plates when the prosthesis is not stressed.
Thus, the body is in contact with at most one of the two plates, or with neither of them. This therefore reduces the friction between the body and the plates, and also the wear and the generation of particles.
The body is advantageously immersed in the fluid.
The body is advantageously movable relative to each plate.
This characteristic further reduces the probability of friction between the body and the plates, as the body spontaneously arranges itself in a position in which it is stressed to the least possible extent by the plates.
The fluid is advantageously compressible.
Thus, the stresses created by the relative movements of the plates are not only returned across the whole surface of the body and in all directions by the fluid. Said fluid also takes up some of these stresses itself, the remainder of said stresses being taken up by the compressible body.
The fluid advantageously has a resistance to compression less than that of the body.
The body advantageously has at least one cell isolated from the outside of the body.
The presence of one or more cells influences the mechanical behavior of the compressible body over and above the choice of the material and its dimensions.
The cell is advantageously filled with a second fluid.
When the prosthesis is unstressed, the second fluid advantageously has a pressure greater than or equal to that of the fluid in the chamber.